The present invention relates to the field of semiconductor manufacturing. More specifically, the present invention relates to a process for the realization of at least partially relaxed strained material.
III-V materials have recently become the center of much research due to a vast range of applications in which they outperform the classic silicon wafers. For instance, III-V materials have excellent performances for optoelectronic, photovoltaic and power applications, such as photovoltaic cells, particularly multi-junction photovoltaic cells, laser diodes, LED, diodes and many more.
Unlike silicon, however, realizing bulk wafers of III-V materials is very expensive or, in some cases, not possible. In those cases, one possible technique for obtaining bulk structures of III-V materials is to grow them epitaxially from a seed substrate.
For instance, as shown in FIG. 6, it is possible to use a donor substrate with strained III-V epitaxial layers, one III-V layer (GaN) and an additional strained layer 6130 (InGaN) on which a compliant or low viscosity layer 6120 made of borophosphosilicate glass BPSG, that is, SiO2 that contains for example 4.5% of boron and 2% of phosphorous and has a glass transition temperature about 800° C., is deposited, or another compliant material. The strained layer is transferred on a new intermediate substrate 6110 via the borophosphosilicate layer, or another compliant material, by SmartCut™ processing resulting in structure 6100. When carrying out a relaxation step for instance by increasing the temperature of the structure 6100, the strained layer can be partially relaxed by flowing of the low viscosity layer, the flowing being a plastic deformation in contrast of the elastic relaxation of strained islands.
Such a relaxation step S61, however, suffers from a problem known as buckling. More specifically, the resulting at least partially relaxed layer of InGaN 6231 has an undulated shape, due to the strain of the lattice being released at least partially in direction D1. Furthermore, in structure 6200, the layer 6221 may also be buckled.
The above mentioned buckling problems can be reduced by a manufacturing process such as the one illustrated in FIG. 7 and known from patent document EP2151852A1. As can be seen in FIG. 7, a structure 7100 includes a support substrate, e.g. of Si, SiC, Ge or sapphire, 7110, a compliant or relaxing layer e.g. BPSG, an SiO2 compound comprising B (BSG) or P (BPG), 7120 and islands of strained material, e.g. InGaN, 7130. The structure 7100 is then subjected to a relaxation step S71 by heating the structure 7100 and flowing of the compliant layer. The resulting structure 7200 includes islands 7231 of at least partially relaxed strained material. The at least partial relaxation of the islands 7231 results in an elongated compliant layer 7221, at least at the interface with the islands 7231.
During a further deposition step S72, a buried layer 7340 is deposited on the islands 7231 of at least partially relaxed material. On top of the buried layer 7340, a second support substrate 7350 is subsequently bonded to bury layer 7340. The resulting structure 7300 therefore comprises islands 7231 of at least partially relaxed material which are connected to both the first support substrate 7110 via layer 7221 and the second support substrate 7350 via layer 7340. During a subsequent transfer step S73, the structure 7400 including the islands 7231 of at least partially relaxed material, the buried layer 7340 and the second support substrate 7350 is detached from structure 7300. The detachment can be achieved, for instance, by laser lift off that comprises irradiation of an absorbing layer between the substrate and the islands 7231 with a laser.
Alternatively, the detachment could be achieved by removing both the support substrate 7110 and the elongated seed layer 7221, for instance, by Chemical Mechanical Polishing (CMP) or also by implantation ions in the seed layer 7120 and by subsequently heating structure 7300.
At this point, an epitaxial growth of at least partially relaxed structures 7560, by using the islands 7231 as seeds, can be carried out in a step S74. Also, by controlling the amount of relaxation during step S71, the physical characteristics such as lattice parameter, low dislocation density, degree of relaxation, of the at least partially relaxed structures 7560 can be controlled.
Such a process, however, still has some drawbacks. It requires a high number of steps and the buckling phenomenon can still occur depending of the size of the islands.
Moreover, when the strain of the strained material is high, cracking and delamination of the strained InGaN can be observed. The strain may depend on the lattice mismatch between the seed layer and the islands and the thickness of the islands. The higher the thickness, the higher is the strain. In the case of an InGaN island and GaN seed layer, the higher the amount of Indium in InGaN the higher the lattice mismatch and strain.
There therefor is a need to improve these prior art processes by reducing the amount of steps and by further reducing the buckling phenomenon. The present invention now satisfies this need.